Tag-use antenna and tag using the same

ABSTRACT

The present invention relates to a tag-use antenna allowing a miniaturization while maintaining a constant minimal change of a communication distance. The tag-use antenna has a feed part of a folded dipole antenna of a size of 53 mm long and 7 mm wide being connected to, and equipped with, an LSI chip of Rc=500 ohms and Cc=1.4 pF and is covered with plastic resin  13  of the dielectric constant εr=3 and thickness of t=0.75 mm on both sides of the antenna. The dipole part of 1 mm wire path width of the tag-use antenna is formed in a rectangular spiral by being bent inward from both ends at bending parts at four places. The entire length of the dipole antenna when extending the four bending parts straight is featured so as to be shorter than one half of a resonance wavelength of the antenna. An inductance part is formed in the neighborhood of the center of the dipole antenna, and placed in the middle of the dipole antenna, which is formed in said rectangular spiral, of the dipole antenna

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/513,188, filed Aug. 31, 2006, which is hereby incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ultra-miniature, tag-use antennaused for a large-scale integration (LSI) chip and a tag using it for usein a radio frequency identification (RFID) system which is capable ofcarrying out a communication between a reader/writer and a tag by usinga radio high frequency signal.

2. Description of the Related Art

An RFID system is one for transmitting a signal of approximately onewatt from a reader/writer (simply “RW” hereinafter), receiving thesignal on a tag side and retransmitting a response signal back to the RWside, by using an ultra high frequency (UHF) between 860 MHz and 960MHz, thereby enabling the RW to read information stored in the tag.

The tag is constituted by a tag-use metallic antenna formed on a flatsurface such as a sheet or a film, et cetera, of approximately 0.1 mmthick, and an LSI chip connected to a feed point of the tag-use antenna.The LSI chip is usually smaller than a sesame seed, actuallyapproximately 0.2 mm thick and its area size is approximately 1 mmsquare.

A communication distance between the RW and tag is about 3 to 5 m,depending on a gain of a tag-use antenna, an operation voltage of a LSIchip, and an environmental condition of the surrounding, et cetera.

FIGS. 1A, 1B and 1C are diagrams respectively describing tag-useantennas used for a conventional RFID system. FIG. 1A shows a tag-useantenna comprising dipole parts 2 extending horizontally on both sidesof a power feed part 1; FIG. 1B shows one having folded dipole parts 3having both ends of FIG. 1A turned back; and FIG. 1C shows one having aninductance part 4 connected in parallel with the dipole parts 2 to thefeed part 1 shown in FIG. 1B.

FIG. 2 is a diagram showing an equivalent circuit of the tag-use antennaand LSI chip used for an RFID system, with the left side showing anequivalent circuit 5 of the tag-use antenna and the right side showingan equivalent circuit 6 of the LSI chip.

FIG. 3 is a diagram exemplifying an analysis, by an admittance chart, ofa tag using a conventional tag-use antenna. An admittance chart isindicated by zero (“0”) ohm on the left side of a pure resistance line,which divides the circle of the chart into the top and bottom half, andinfinite (“^(∞)”) ohms on the right side thereof.

As shown in FIG. 2, a tag-use antenna can be equivalently expressed by aparallel connection of an emission resistance Ra and of an inductanceLa, while an LSI chip can be equivalently expressed by a parallelconnection of a resistor Rc and of a capacitance Cc.

Then, the parallel connection of the tag-use antenna and LSI chip makesthe inductance La and capacitance Cc resonate, and they match at adesired resonance frequency f0 as is apparent from an expression“f0=1/(2π√(LC) )”, resulting in a reception power at the tag-use antennabeing adequately supplied to the LSI chip side.

That is, letting an emission resistance Ra of the tag-use antenna be 400ohms for example, a resistance Rc of the LSI chip be 500 ohms, forexample, a configuration be so as to cancel out resistance of both, andassuming L=La=20 nano Henry (abbreviated as “nH” hereinafter) andC=Cc=1.4 pF in the above noted expression of the resonance frequency,then a desired resonance frequency of f0=953 MHz required for an RFIDsystem is obtained.

For a basic antenna used for a tag-use antenna, first conceivable is adipole antenna of a whole length of about 145 mm which is constituted bydipole parts 2 extending horizontally in both direction of the feed part1 shown by FIG. 1A.

In this configuration, the feed part 1 connected to the dipole parts 2extracts a power from a signal received at the dipole parts 2 and feedthe power to the LSI chip equipped on the feed part 1 and also transfersthe signal per se to the LSI chip. The configuration of the dipoleantenna actually measures an emission resistance Ra=72 ohms.

Incidentally, impedance of an LSI chip of the above noted resistanceRc=500 ohms and capacitance Cc=1.4 pF is indicated at a positiondiagonally on the right below in the direction of about “−40 degrees” ofan ωC zone in the admittance chart (FIG. 3 shows the position simply bya circular plot pointed as “chip”).

In this case, an optimum position, in the admittance chart, of thedipole antenna resonating with the above described LSI chip is aposition of the LSI chip symmetrically reversed relative to the pureresistance line of the admittance chart, and FIG. 3 shows the positiondiagonally on the right above in the direction of about “+40 degrees” ofan ωL zone.

This position is one for an impedance with an emission resistance Ra=500ohms and an inductance La=20 nH (FIG. 3 shows the position by a circularplot indicating “the most optimum position”).

As such, an emission resistance Ra required for an RFID tag-use antennacorresponding to an LSI chip of resistance Rc=500 ohms and capacitanceCc=1.4 pF is very high, i.e., about 500 ohms, and therefore the emissionresistance Ra=72 ohms of the dipole antenna shown by FIG. 1A is far toosmall.

It is accordingly necessary to increase an emission resistance Ra up toabout 500 ohms by devising a configuration of the dipole antenna. Thendevised is a folded dipole antenna having a folded dipole part 3 of awhole length of 145 mm folding back from the both ends of FIG. 1A, asshown by FIG. 1B.

This configuration makes it possible to increase an emission resistanceRa. This configuration is known to allow setting of emission resistancein the range of about 300 to 1500 ohms, depending on a wire width of thefolded part.

FIG. 3 shows an impedance position of a folded dipole with an emissionresistance Ra being 400 ohms indicated by a triangle on the pureresistance line.

Here, with the emission resistance Ra being maintained at 400 ohms, afurther connection of an inductance part 4 to the feed part 1 of FIG. 1Bparallelly with the dipole part 2 as shown in FIG. 1C rotates theantenna characteristic counter-clockwise on the admittance chart.

This results in positioning, close to the most optimized position, theantenna characteristic of the folded dipole antenna, connected with theinductance L, having a resonance frequency of 953 MHz as shown by atriangle as the folded dipole connected with the inductance L (the“L-connected folded dipole antenna” hereinafter) in the ωL zone of FIG.3.

The admittance chart shown by FIG. 3 exemplifies characteristic between700 and 1200 MHz. In the range of the resonance frequency, it isapparent that the antenna characteristic locus 7 of the L-connectedfolded dipole antenna circles around the resonance most optimum value(i.e., the most optimized position of Ra=500 ohm and La=20 nH).

That is, it is apparent that the configuration of the L-connected foldeddipole antenna shown by FIG. 1C widens a frequency band resonating withthe LSI chip.

Incidentally, an RFID is used by being attached to various bodies as atag. In the case of such a body being a styrofoam, the dielectricconstant εr of the RFID is approximately 1.1 which is about the same asthe value in the air (εr=1).

That is, in the case of attaching a tag onto a styrofoam, it becomesabout the same as floating the tag in the air.

In the case of a body attached with an RFID being a plastic for example,an effective dielectric constant around the antenna becomes large if thethickness of the plastic is 2 mm, since the dielectric constant εr of aplastic material is about εr=3.

Meanwhile, a behavior of the RW communicating with an RFID at theoperating frequency 953 MHz is empirically known to be approximately thesame as the characteristic at 953 MHz in the air displaced by 100 MHz.

As such, a practicality is hampered if a communication distance of theantenna fluctuates when being attached onto various kinds of bodies,that is, when the operating frequency is displaced, and thereforedesired is an antenna whose communication distance does not change evenif it is attached to various kinds of bodies.

Therefore, a good antenna for an RFID is one capable of having a widefrequency band characteristic, that is, having a wide frequencycharacteristic.

The L-connected folded dipole antenna, shown by FIG. 1C, comprising theantenna characteristic as shown in FIG. 3 has an adequately wide band,e.g., the bandwidth of the one rotation part 7 a according to theantenna characteristic locus 7 of FIG. 3 is approximately 200 MHz, canbe characterized as a good antenna whose communication distance beinghard to fluctuate due to a material to be attached to (i.e.,uninfluenced by a material to be attached to).

However, there is a strong demand by users for miniaturizing the RFID.An antenna with a size of 145 mm horizontal by 15 mm vertical is toolarge for a tag use. It may be just possible to use it for managing abook for example, while there is no degree of freedom by a limitation ofits usage in terms of other practicalities, thus requiring furtherminiaturization.

Incidentally, if one tries to confine the entire size of an antenna to80 by 20 mm for example, he must bend the antenna line in a serpentinefashion (or “meandering”) in order to house the elongated line lengthinto a small area size.

It is, however, known that a miniaturization of an antenna widens afrequency interval (e.g., the one rotation bandwidth becomesapproximately a mere 20 MHz) of the characteristic part (i.e., thecharacteristic locus 7 a) that rotates one revolution as shown in FIG.3.

That is, the miniaturization of an antenna narrows a frequency band. Inother words, an RFID comprising such miniaturized antenna changescommunication distances drastically depending on the material to beattached to. This is faced with practical problems.

SUMMARY OF THE INVENTION

A tag-use antenna according to the present invention is one comprising adipole antenna, a feed part and an inductance part, which are featuredby a conductor in the same flat plane, wherein the feed part is featuredat the center of the dipole antenna in a manner capable of equippingwith a chip, the inductance part is connected to the feed part inparallel with a dipole of the dipole antenna, and the dipole antenna isformed in a square spiral by being bent inward from both ends at bendingparts which bend the dipole at least in four places respectively, withthe entire length of each of the bending parts of the four places beingshorter than one half of a resonance wavelength of the antenna when thebending parts are extended to a straight line.

The tag-use antenna is configured in such a manner that the inductancepart is featured in the neighborhood of the center of the dipole antennaand placed in the middle of the dipole, which is formed in the squarespiral, of the dipole antenna. The tag-use antenna is configured in sucha manner that the entire length of the antenna and the inductance partare adjusted so as to make an impedance of a tag at a reader/writeroperation frequency of 953 MHz come close to an antenna's most optimumvalue, for example. The conductor is either copper, silver or aluminum,for example.

The tag-use antenna is configured so that the feed part is connected to,and equipped with, a large scale integration (LSI) chip. In this case aconfiguration maybe so as to sandwich the tag-use antenna by a plasticresin or paper from both surface of the tag-use antenna. In this casethe plastic resin is preferably an ethylene terephthalate film.

A tag according to the present invention is configured to sandwich thetag-use antenna by plastic resin or paper from the both sides.

The present invention is contrived to enable a provision of an extremelycompact, tag-use antenna of which a communication distance is changedlittle by a body on which the antenna is attached to, that is, theantenna maintaining a minimally changed communication distance, and of atag using the tag-use antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C are diagrams describing tag-use antennas used for aconventional RFID system;

FIG. 2 is a diagram showing an equivalent circuit of a tag-use antennaand of an LSI chip which are used for an RFID system;

FIG. 3 is a diagram exemplifying an analysis of a tag using aconventional tag-use antenna;

FIGS. 4A and 4B are diagonal view diagrams showing a configuration of anextremely compact tag-use antenna according to an embodiment;

FIG. 5 is an admittance chart showing an impedance characteristic of atag-use antenna according to an embodiment;

FIG. 6 is a diagram showing a reflectance frequency characteristic of atag-use antenna calculated by an electromagnetic field simulatoraccording to an embodiment;

FIG. 7 is a diagram showing a calculation value of an antenna gain of atag-use antenna calculated by an electromagnetic field simulatoraccording to an embodiment; and

FIG. 8 is a communication distance characteristic chart which can obtaina reflectance characteristic and a gain characteristic of a tag-useantenna in an Excel chart according to an embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is a detailed description of the preferred embodiment ofthe present invention by referring to the accompanying drawings.

FIGS. 4A and 4B are diagonal view diagrams showing a configuration of anextremely compact tag-use antenna according to an embodiment. Note thatFIGS. 4A and 4B show a tag-use antenna along with a tag built in withthe tag-use antenna. As shown in FIG. 4B is an enlarged version of FIG.4A, in which a sandwich structure of the tag-use antenna was enhancedfor viewing purpose. Here, the tag is configured by sandwiching bothsides of the tag-use antenna by a plastic resin or paper. FIGS. 4A and4B indicate a tag-use antenna internally by perspectively showing theplastic or paper tag.

The entire size of the tag-use antenna 10 shown in FIGS. 4A and 4B is 53mm horizontal by 7 mm vertical.

The tag-use antenna 10 comprises a dipole antenna, a feed part and aninductance part which are featured by a conductor within the same plane.The conductor preferably uses cupper, silver or aluminum.

The feed part is formed to enable an equipment of an LSI chip at thecenter of the dipole antenna and comprises a chip equipment part 8 asshown in FIGS. 4A and 4B. With the chip equipment part 8 being at thecenter, the either side is featured a dipole part 9 of a line path widthof 1 mm, thereby featuring a dipole antenna as the whole.

The dipole antenna constituted by the dipole part 9 on both sides isformed in a rectangular spiral by bending the dipoles inward from bothsides at bending parts 11 (i.e., 11-1, 11-2, 11-3 and 11-4) in at leastfour places. That is, the present embodiment has four bending parts oneach side.

The entire length of the dipole antenna is featured shorter than onehalf of a resonance wavelength of the antenna, as described later indetail, when the respective bending parts are extended in straightlines.

In the neighborhood of the dipole antenna is featured an inductance part12 in the intermediate part of both dipole parts 9 and 9 which arerespectively formed in the above described rectangular spirals. Theinductance part 12 is connected to the chip equipment part 8, that is,the feed part of the antenna, parallelly with the both dipole parts 9and 9.

The dipole antenna (i.e., the tag-use antenna 10) is comprised as a tagby connecting, and equipping, an LSI chip of Rc=500 ohms and Cc=1.4 pF,to and with the feed part (i.e., the chip equipment part 8), and bothsurfaces (i.e., the top and bottom surfaces in the showing of FIGS. 4Aand 4B) are covered by plastic resin 13 of the dielectric constant εr=3and thickness t=0.75 mm.

The plastic resin 13 uses an ethylene terephthalate film, et cetera, forexample. Or, a suitable paper may be used for covering the both surfacesin place of the plastic resin 13.

FIG. 5 is an admittance chart showing an antenna characteristic of thetag-use antenna 10, that is, an impedance characteristic thereof. Notethat the chip position and the most optimum position of the antenna inthe showing of FIG. 5 are the same as the case of FIG. 8.

The admittance chart shown by FIG. 5 shows a locus 14 indicating anantenna characteristic as a result of inputting values of Ra, La, Rc andCc of the tag-use antenna 10, as shown in FIG. 7, to a commerciallyavailable electromagnetic field simulator and calculating impedance inthe frequency band between 800 and 1100 MHz.

The antenna characteristic locus 14 revolves around the most optimumvalue of the antenna, with the locus 14 being the closest thereto in theneighborhood of 953 to 1000 MHz of the RW operation frequency which isindicated by enclosing by the dotted line eclipse 15 in FIG. 5. That is,a reflectance between the LSI chip and dipole antenna is small.

Further increasing frequency so as to approach a position exceeding 1050MHz indicated by a dotted line eclipse 16 in FIG. 5, a reflectancebetween the LSI chip and dipole antenna becomes large because it is farfrom the most optimum value of the antenna.

FIG. 6 is a diagram showing a frequency characteristic of a reflectionS11 of the tag-use antenna 10 calculated by the above notedelectromagnetic field simulator, showing frequencies (800 MHz through1100 MHz) on the horizontal axis and reflections S11 (−5 dB to 0 dB) onthe vertical axis. As understood from FIG. 6, the reflection S11indicates a minimum at around 975 MHz.

FIG. 7 is a diagram showing a calculation value of an antenna gain ofthe tag-use antenna 10 calculated by the above noted electromagneticfield simulator, showing frequencies (800 MHz through 1100 MHz) on thehorizontal axis and antenna gains (−4 dBi through 2 dBi) on the verticalaxis. The antenna gain shown in FIG. 7 indicates a maximum at around1050 MHz.

That is, although there is a shortfall of the reflection becoming largeat around 1050 MHz as shown in the admittance chart of FIG. 5, theantenna gain is large at around 1050 MHz as understood from FIG. 7,resulting in the large antenna gain compensating the shortfall of alarge reflection.

FIG. 8 is a communication distance characteristic chart which can beobtained by combining the above noted reflectance characteristic and again characteristic of a tag-use antenna in an Excel chart, showingfrequencies (800 MHz through 1100 MHz) on the horizontal axis andrelative communication distances specified by a maximum distance on thevertical chart.

As described above, a communication distance characteristic possessed bythe tag-use antenna 10 is asymmetrical in the left and right directionsrelative to the RW operation frequency of 953 MHz, with the gainchanging gradually on the higher frequency side of the RW operationfrequency of 953 MHz and a characteristic of being relatively stablecommunication distance.

The above noted calculation by the electromagnetic field simulatorspecifies the top and bottom of the plastic resin 13 shown in FIG. 4 asthe air and therefore the communication distance at the RW operationfrequency of 953 MHz is a distance when the tag-use antenna is in theair. The communication distance in the air is a distance of 0.95 asopposed to the specified maximum distance as shown in FIG. 8. That is, a95% of the maximum distance is secured.

When attaching the tag-use antenna 10 to a plastic of εr=3 and 2 mmthick, an effective dielectric constant around the antenna becomeslarge, down-shifting a band approximately by 10%. That is, the waveformshown in FIG. 8 shifts toward lower frequency side by approximately 100MHz.

In other words, the value of the relative communication distance at 1050MHz, which is higher than 953 MHz by about 10% becomes a communicationdistance when attaching the tag-use antenna onto a 2 mm thick plasticaccording to the waveform shown in FIG. 8. The communication distance inthis case is a distance of 0.8 as opposed to the specified maximumdistance as shown in FIG. 8, securing 80% of the maximum distance.

As is also apparent from FIG. 8, the tag-use antenna 10 according to thepresent embodiment is configured to constantly secure within 80% of themaximum communication distance in the air, when being attached to astyrofoam or to a 2 mm plastic, and hence possess an extremely high,distance stability.

A remarkable characteristic of the tag-use antenna according to thepresent embodiment is that the antenna pattern constituted by the dipolepart and inductance part is adjusted in a manner to approach the mostoptimum value of the antenna in the neighborhood of the RW operationfrequency of 953 MHz, while reflectance becomes large as departing fromthe most optimum value in higher frequencies than 953 MHz, which iscompensated by higher antenna gain, resulting in keeping thecommunication distance at a minimal loss.

In order to obtain higher antenna gains in higher frequency than 953MHz, the entire length of the antenna is configured to be close to onehalf of the antenna resonance wavelength providing good gain efficiency.

The antenna pattern of the tag-use antenna 10 according to the presentembodiment is remarkably characterized by configured in such a mannerthat the entire length of the antenna is a little shorter than one halfof the antenna resonance wavelength λ when the bending parts 11 areextended straight.

The example shown by FIG. 4 is configured that the entire length of theantenna is approximately 120 mm when extending the bending partsstraight, while one-half of the antenna resonance wavelength λ isapproximately 130 to 140 mm, with the 10 mm tolerance band of theantenna resonance wavelength λ comprehending the plastic resin 13 on thetop and bottom sides.

Meanwhile, the dipole part is maintained as straight as possible bybending from the side to inside, and the inductance part is desirablyfeatured between both dipole parts because they are not to approach eachother.

By this configuration, impedance at 953 MHz is set to approach the mostoptimum value of the antenna and to maximize the antenna gain in theneighborhood of 1050 MHz as shown in FIG. 5.

This configuration enables an accomplishment of a tag-use antennapossessing an extremely high distance stability always securing adistance within 80% of the maximum communication distance either in theair, or by being attached to a styrofoam or a 2 mm thick plastic.

Note that the present invention assumes that a size of the entiretag-use antenna (i.e., an L-connected dipole antenna) be 30 to 80 mmhorizontal by 6 to 15 mm vertical.

While four bending parts are formed on either of both dipole parts forthe size of 53 mm horizontal by 7 mm vertical as shown in FIG. 4, thenumber of bending parts will be increased to five or six, and so on, asan antenna becomes smaller.

As described thus far, the present invention is contrived to enable aprovision of a tag-use antenna and a tag which allow a minimal change ofa communication distance depending on a body to be attached to by usingan extremely compact antenna.

1. A tag-use antenna comprising a dipole antenna, a feed part and aninductance part formed by a conductor in the same flat plane, whereinthe feed part is formed at the center of the dipole antenna and is ableto mount a chip, the inductance part is connected to the feed part inparallel with a dipole of the dipole antenna, and the dipole antenna isformed in a rectangular spiral by being bent inward from both ends atbending parts which bend the dipole at least in four placesrespectively, with the entire length of each of the bending parts of thefour places being shorter than one half of a resonance wavelength of theantenna when the bending parts are extended to a straight line, whereinthe four bending parts are formed on either of both dipole parts for asize of 53 mm horizontal by 7 mm vertical, a number of bending parts isincreased to more than four to make the dipole smaller, wherein theinductance part is formed in the neighborhood of the center of thedipole antenna, and placed in the middle of the dipole antenna, which isformed in said rectangular spiral, of the dipole antenna.
 2. The tag-useantenna according to claim 1, wherein the entire length of the antennaand said inductance part are adjusted to make an impedance of a tag at areader/writer operation frequency of 953 MHz nearly become an optimumantenna value, wherein a reflectance between the chip and the dipoleantenna is small.
 3. The tag-use antenna according to claim 1, whereinsaid conductor is either copper, silver or aluminum.
 4. The tag-useantenna according to claim 1, wherein said feed part is connected to amounted large scale integration (LSI) chip.
 5. A tag for sandwiching thetag-use antenna according to claim 4 which uses plastic resins or papersattached to both surface of the tag-use antenna.
 6. The tag according toclaim 5, wherein said plastic resins are ethylene terephthalate films.